1. Field of the Invention
This invention is directed to probes for inspecting the interior walls of cavities in electrically conductive objects, more particularly to inspecting the interior walls of hollow metallic plugs inserted in the tubes of heat exchangers and having a cylindrical bore with a radially expanded section and a narrower opening. Specifically, it is directed to a probe with a radially extendable and retractable eddy current detector assembly.
2. Background Information
Eddy current detectors, known in the art, are used to inspect the tubes of heat exchangers, and in particular, the tubes of nuclear steam generators. The eddy current detector consists of a pancake-style eddy current coil mounted in an inspection probe. The probe is inserted into a steam generator tube at the tube sheet, and gradually moved upward while being rotated inside the tube, thus inspecting the integrity of the tube in a helical pattern. The pancake coil is typically mounted in a spring-loaded housing which is designed to allow the pancake coil to extend and swivel slightly to remain in contact with the tube wall in out-of-round sections or other none-uniform areas.
In addition to inspecting tubes, it has become necessary to inspect "roll plugs" installed in out-of-service tubes to ensure the integrity of the plug. Roll plugs are secured in the out of service tube by radially expanding a section of the bore in the plug. This creates a "roll zone" which is somewhat larger in diameter than the remainder of the bore in the plug. This roll zone is spaced from the opening in the roll plug. Consequently, the eddy current detector used to inspect the roll plug must be mounted in the housing which extends the detector into this larger-diameter area to maintain contact with the walls in the roll zone. A previous type of probe incorporated an eddy current detector which was simply spring-loaded, thereby insuring that the detector would extend to the plug wall in the roll zone. However, this probe was sometimes destroyed upon an insertion into a roll plug when the extended spring loaded detector housing caught at the opening and was torn out of the probe. An example of an eddy current detector which is merely spring biased outward from a probe is disclosed in U.S. Pat. No. 4,625,165. U.S. Pat. No. 4,992,735 discloses a probe with an eddy current detector mounted on the end of a leaf spring which is retracted axially into a probe housing to protect the detector while the probe is maneuvered past a retainer ring in the plug. This device requires a special, sizable end effector for a robot which performs the inspection.
There remains a need for an improved eddy current detector probe for inspecting cavities in electrically conductive objects with narrowed openings, and in particular for inspecting roll plugs for steam generator tubes.
There is a further need for such an improved probe which retracts the eddy current detector within the probe body for insertion and retraction of the probe, yet biases the detector against the surface of the object to be inspected when in use.
There is a preferred need for a probe which automatically extends and retracts the eddy current detector to eliminate operator error in failing to retract the probe.
There is also a need for providing improved probes as described above which can be used with standard eddy current inspection systems and with minimum requirements for modifications to the inspection system.